The present invention relates to a contrast sensitivity measuring device suitable for use in a visual acuity test using a contrast chart. More specifically, the present invention relates to a contrast sensitivity measuring device and a contrast sensitivity measuring method suitable for precisely determining the ocular aberration of a subject at the time of a laser optical surgery or improvement of visual acuity with glasses or contact lenses.
In recent years, optical surgeries by which the shapes of corneas are altered with laser are attempted. At the time of such surgeries, it is important to precisely measure the ocular aberration. Before and after a cornea surgery or a cataract surgery, visual function tests, including measurement of visual acuity, a low-contrast visual acuity test (which is also referred to as “contrast visual acuity test”) and a contrast sensitivity test are performed.
Here, a general visual acuity test using a standard visual acuity chart measures the visual acuity to a minute object with a high contrast ratio. Then, in a cataract patient, for example, turbidity of a lens causes scatter of light, which lowers the contrast of an image on the fundus but does not affect deviation of the optical focus position of the image. As a result, the cataract patient as a subject may be able to read the image as a letter although the image on the fundus is blur. Also in the case where a spherical aberration is caused by optical distortion of a cornea and a lens, the subject may be able to read the image as a letter although the image on the fundus is blur.
Contrast sensitivity which represents the ability to detect a specific object with a minimum contrast is an important index to express the visual ability of a subject in daily and social life. Thus, in measurement of contrast sensitivity, a sine wave grid pattern is used to examine a single modulation transfer function. A sine wave grid pattern is a grid pattern in which the density is changed in a sine wave pattern. In a contrast sensitivity test, the minimum contrast necessary to recognize the grid pattern is measured at various finenesses (spatial frequencies).
However, since contrast sensitivity is measured in such a manner that a subject reads a visual acuity chart under a examiner's direction, there are the following problems.
(1) As the test is performed using a single visual acuity chart, the subject remembers the visual acuity chart when the test is repeated. For example, when the left eye is examined after the right eye and when the right eye is better than the left eye, the subject can respond correctly with memory even if it cannot see with the left eye. Thus, it is difficult to measure the contrast sensitivity of a subject precisely.
(2) It takes about 10 minutes to measure contrast sensitivity and a clinical technologist or ophthalmologist must concentrate on the test during that time. Thus, the practice time or the examination time is unavoidably long.